The field of the invention is imaging systems used in the practice of nuclear medicine, and more specifically to the determination of a cardiac ejection fraction from a sequence of nuclear images of a heart.
Nuclear imaging systems such as those described in U.S. Pat. Nos. 4,497,024 and 4,652,758 include a sensor, or camera, which is sensitive to emissions from radioactive substances introduced into a patient. The camera can be held stationary to produce a single anatomical view or rotated about the patient to produce a series of views from different angles. To produce each view, the camera senses the magnitude of the radiation received from the patient at a two-dimensional array of sensing points, thereby producing an equivalent number of data samples. The relative magnitude of each radiation sample can be used as a picture element to construct a two-dimensional image of the patient anatomy.
Images produced by these cameras are used in cardiology to evaluate heart function. In performing this evaluation, the radiologist injects a radio pharmaceutical into the patient's blood stream. The substance emits gamma rays as it is carried throughout the patient's body by the blood flow. During the cardiac cycle, the heart fills with blood thereby concentrating a significant amount of the radio pharmaceutical in the heart cavities. The images produced by counting the gamma radiation emanating from the chest of the patient clearly show the heart cavities which can easily be distinguished from blood vessels and other organs. A series of these images often is produced depicting the heart at different stages of the cardiac cycle.
From this series of images, the cardiac ejection fraction can be calculated as one parameter of heart performance. The cardiac ejection fraction is the average fractional decrease in blood volume of the heart as it beats. An example of a system for deriving this parameter is described in a paper by J. H. C. Reiler et al. entitled "Clinical Validation of fully Automated Computation of Ejection Fraction from Gated Equilibrium Blood-Pool Scintigrams" which appeared in Volume 24, page 1099 of The Journal of Nuclear Medicine (1983). In order to calculate the cardiac ejection fraction, the blood volumes of the heart's left ventricle is derived at the diastole and the systole of the cardiac cycle. The initial step in finding these volumes involves filtering the image data samples and using conventional edge detection and contour extraction techniques to determine the perimeter of the heart chambers. Several well known pattern recognition techniques have been used to locate the left ventricle perimeter in the image. The data samples within the perimeter of the left ventricle then are summed to provide a numerical value proportional to the left ventricle blood volume in that image, once extraneous background artifacts affecting the data samples have been taken into account.
The most accurate method for determining the cardiac ejection fraction requires deriving the left ventricle blood volume indication for every image taken during a cardiac cycle. The image for which the sample sums are the largest and the smallest depict the heart at diastole and systole, respectively. As 16 to 32 images typically are taken during a cardiac cycle, with more images providing greater accuracy, deriving the ejection fraction becomes a very time consuming, non-real time process.
As a consequence, an approximation technique is often used in place of individually calculating the left ventricle count sum for each cardiac cycle image, in order to decrease the evaluation time. For this technique, the QRS complex or the R-wave of an electrocardiogram signal produced by heart activity is employed as a reference point for initiating the acquisition of images. By properly timing the image acquisition with respect to this signal, the first image acquired usually depicts the heart near or at the diastole. The perimeter of the left ventricle of the first image is determined and the gamma radiation count samples which lie within that perimeter are summed. This sum is assumed to represent the diastolic blood volume. Instead of determining the left ventricle perimeter for each image in the series, the perimeter from the first image is used to select the count samples to sum for each remaining image. The smallest sum is considered to be the systolic blood volume and is used with the sum for the first image in computing the ejection fraction.
Although this latter process provides a reasonably accurate approximation of the ejection fraction, some discrepancy can exist between the approximated fraction and the actual value. For example, the first image in the series may not have been taken when the actual diastole occurred. In addition, since the left ventricle perimeter of the first image is used in deriving the count summation in subsequent images, other heart chambers and anatomical elements can enter the area defined by that perimeter as the heart contracts in those images. In this case, the count sum for the true systolic image may include count samples from these elements and have a greater magnitude than another image. Thus, the approximation technique may select the incorrect image as occurring at the systole.